For next-generation optical communication, a reconfigurable optical add/drop multiplexer (ROADM) network has been proposed. Demands of the ROADM include flexible coping of dynamic traffic variation, unexpected updating of the network configuration, etc. Optical paths differing signal bit rates and differing modulation formats, each of which differ depending on the required transmission capacity and the transmission distance. Thus, the spectrum bands used by the optical paths differ from each other.
Consequently, optical communication systems have started to employ a flexible grid dense wavelength division multiplexing (DWDM) scheme instead of a conventional fixed grid wavelength division multiplexing scheme.
Distributed feedback (DFB) array wavelength-tunable lasers are present as wavelength-tunable light sources that can arbitrarily vary the wavelength output and that are employed in optical communication systems. The DFB array includes, for example, a laser array unit, an optical coupling unit, and a semiconductor optical amplifier (SOA) (see, e.g., Japanese Laid-Open Patent Publication No. 2011-198903). The DFB array wavelength-tunable light sources do not cause mode hopping (discreteness of the frequencies) that a device employing an outer oscillator (a diffraction grating) causes, and can narrow the linewidth. Therefore, the light source is suitable as a coherent light source, and the size and the cost thereof can be reduced.
The DFB lasers integrated in an array form, e.g., 12-DFB laser array, are designed such that adjacent lasers have oscillation wavelengths that differ from each other by 3.2 nm. A laser is operated, whereby the oscillation wavelength is roughly selected. The temperature of the selected laser chip is varied, whereby the oscillation wavelength is adjusted. For an ordinary DFB laser array, a Peltier cooler, etc. is disposed immediately beneath the integrated DFB array element and thereby, adjusts the temperature of the entire DFB array. With this configuration, for example, a band of 4.8 THz (=12×400 GHz) can be varied that is formed by 80 wavelengths in C band at 50-GHz intervals.
However, for the DFB laser array, for example, concerning the variation of the wavelength of only one of the 12 lasers, the wavelength of the laser can be caused to continuously vary by causing the temperature to continuously vary. However, to increase the range of the variation of the wavelength, another (an adjacent) laser has to be selected and the laser in operation has to be switched. Consequently, when the laser is switched, the output of the light becomes discontinuous and smooth variation of the continuous wavelength can not be achieved for a range that exceeds the range within which the wavelength can be varied by the temperature variation of one laser device.
With the above flexible grid scheme, unusable fragmentary spectrum bands appear in the used band (such as C band) after the optical path is repeatedly changed. Thus, a problem arises in that the efficiency of spectrum band use drops. To cope with this, wavelength defragmentation is proposed in which unused spectrum bands are filled by gradually varying the wavelength while maintaining the connection of the optical path that is currently in operation. However, with the DFB laser array, smooth variation of the continuous wavelength can not be achieved and the optical output is discontinuous and therefore, wavelength defragmentation can not be executed and no improvement of the efficiency of wavelength use can be facilitated.